The subject matter of the present invention is in a method for the in-depth hydrophobic impregnation of concrete by the use of alkyl trialkoxysilanes.
It is known from German Patent No. 2,029,446 to impregnate porous building materials, including concrete, with alkyl trialkoxysilanes. The impregnation is performed by applying solutions of the silanes to the surface of the building materials. Alcohols are named as the preferred solvents in this patent, because the best impregnation effect is achieved with these solvents.
The good impregnating effect of alcoholic solutions of the alkylsilanes has heretofore been explained on the basis that they are especially able to penetrate into the material, the alcohol being considered as a particularly good transport medium permitting a deep penetration of the silane into the material.
The use of this known alkyl silane solution in the rendering of concrete hydrophobic, however, has the following disadvantage: since the pore volume of concrete is generally very small, and better quality concrete is of lower pore volume, relatively little alkyl silane solution can penetrate into the concrete in a single application of this solution. The small pore volume in contact with the atmosphere, as is the case in virtually all concrete construction, is more or less filled with water because of exposure to atmospheric water vapor. The alkyl silane solution therefore is unable to penetrate sufficiently deeply into the surface to produce long-term action of the silanes.
The action of alkyl trialkoxysilanes in rendering a surface hydrophobic is known to be based on the fact that, on account of the atmospheric moisture, and in some cases the water that is on the surface, alkyl silanols are formed intermediately, whose hydroxyl groups react with the hydroxyl groups of the material to be rendered hydrophobic, and with other hydroxyl groups of the silanol resulting in the formation of alkyl siloxanes. A bond is thus produced between alkyl siloxanes and the mineral boundary surface. This boundary surface bond greatly increases the boundary surface tension of the mineral materials with respect to water and the body becomes hydrophobic. The degree of the water repellency depends on the nature of the alkyl group. The bond between this alkyl group and the silicon atom of the alkyl silanes, however, can be destroyed by energy-rich radiation, such as ultraviolet light, for example.
The long-term hydrophobic effect obtained with alkyl silanes therefore depends on the extent that light rays and ultraviolet rays are able to destroy the Si--C bond in the system. The more deeply the silane can penetrate into the surface of the body to be protected, the less will be the destructive action of light and ultraviolet rays and the greater will be the long-term effect of the silane.
The prior art thus faced the problem in rendering concrete hydrophobic with alkylsilanes, of finding methods of causing the silane to penetrate deeper into the surface of the concrete. The obvious solution to this problem, repeated application of silane solutions after the solvent has evaporated, requires periods of waiting, is labor-intensive, and causes losses of silane because some silane is carried off by the evaporation of the solvent.
It is already known from DE-AS No. 1,796,012 to apply silanes as well as other substances to the surface of concrete. The object in that case was, however, to make the concrete resistant to spalling and chipping. For this reason the silane is applied to the still unset concrete. In the present case, however, an already set concrete is treated with the silane, so that nothing could have been learned from this "Auslegeschrift" toward the solution of the above described problem.